The present invention relates to the apparatus for measuring the oxygen quantity in objects such as the cerebral tissues of a human body or an animal. The invention especially relates to the apparatus for measuring the oxygenation of hemoglobin in blood and of cytochrome in cells by detecting those through electromagnetic waves.
In general, in diagnosing the function of a body organ such as the cerebral tissues, the fundamental and important parameters to measure are the oxygen quantity in the body organ and the organ's utilization of oxygen. Supplying body organs with a sufficient quantity of oxygen is indispensable for the growth ability of fetuses and new-born infants. If the supply of oxygen to a fetus is insufficient, the probability that the fetus will not survive or that the new-born infant will die is high. Even if the new-born infant lives, however, serious problems in body organs may remain as sequela. The insufficiency of oxygen affects every body organ, but especially causes damage in the cerebral tissues.
To examine the oxygen quantity in body organs readily and at the early stage of illness, an examination apparatus disclosed in U.S. Pat. No. 4,281,645 patented on Aug. 4, 1981 has been developed. In this kind of examination apparatus, the variation of oxygen quantity in body organs, especially in the brain, is measured through the absorption spectrum of near infrared light. The absorption is caused by the hemoglobin which is an oxygen-carrying medium in blood and the cytochrome a, a.sub.3 which performs oxydation-reduction reaction in cells. As shown in FIG. 1(a), the absorption spectra of near infrared light (700 to 1300 nm), .alpha..sub.HbO2 and .alpha..sub.Hb by oxygenated hemoglobin (HbO.sub.2) and disoxygenated hemoglobin (Hb), respectively, are different from each other. And as shown in FIG. 1(b), the absorption spectra of .alpha..sub.CyO2 and .alpha..sub.Cy by oxidized cytochrome a, a.sub.3 (CyO.sub.2) and reduced cytochrome a, a.sub.3 (Cy), respectively, are different from each other. This examination apparatus utilizes the above-described absorption spectra of near infrared light. Four near infrared light rays with different wavelengths, .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 (e.g. 775 nm, 800 nm, 825 nm and 850 nm) are applied to one side of the patient's head with a time-sharing method and the transmission light rays from the opposite side of the head are in turn detected. By processing these four detected light rays with the prescribed calculation program the density variations of oxygenated hemoglobin (HbO.sub.2), disoxygenated hemoglobin (Hb), oxidized cytochrome a, a.sub.3 (CyO.sub.2) and reduced cytochrome a, a.sub.3 (Cy) are calculated. these parameters, in turn, determine the variation of cerebral oxygen quantity.
FIG. 2 shows a system outline of the above-described conventional examination apparatus 45. The conventional examination apparatus 45 includes; light sources such as laser diodes LD1 to LD4 which emit four near infrared light rays with different wavelengths of .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4, respectively; a light source control device 55 which controls output timing of the light sources LD1 to LD4; optical fibers 50-1 to 50-4 which introduces near infrared light rays emitted by the light sources LD1 to LD4 to a patient's head 40; an illumination-side fixture 51 which bundles and holds end portions of the optical fibers 50-1 to 50-4; a detection-side fixture 52 which is fitted to the prescribed position of the opposite side of the patient's head 40; an optical fiber 53 which is held by the detection-side fixture 52 and introduces transmitted near infrared light from the patient's head 40; a transmission light detection device 54 which measures transmission quantity of near infrared light by counting photons of near infrared light introduced by the optical fiber 53; and a computer system 56 which controls the total examination apparatus and determines the variation of oxygen quantity in cerebral tissues being based on the transmission quantity of near infrared light.
The computer system 56 is equipped with a processor 62, a memory 63, output devices 64 such as a display and a printer, and an input device 65 such as a keyboard, and these devices are connected to each other by a system bus 66. The light source control device 55 and the transmission light detection device 54 are connected to the system bus 66 as external I/O's.
The light source control device 55 receives instructions from the computer system 56 and drives the light sources LD1 to LD4 by respective driving signals ACT1 to ACT4 as shown in FIGS. 3(a) to 3(d). As shown in FIG. 3 one measuring period M.sub.k (k=1, 2, . . . ) consists of N cycles of CY1 to CYn. At a phase .phi.n1 in an arbitrary cycle CYn, no light source of LD1 to LD4 is driven and therefore the patient's head 40 is not illuminated by the near infrared light from the light sources LD1 to LD4. At the phase .phi.n2 the light source LD1 is driven and the near infrared light with the wavelength of, for example, 775 nm is emitted from it. In the same manner, at the phase .phi.n3 the light source LD2 is driven and the near infrared light with the wavelength of, for example, 800 nm is emitted from it; at the phase .phi.n4 the light source LD3 is driven and the near infrared light with the wavelength of, for example, 825 nm is emitted from it; and at the phase .phi.n5 the light source LD4 is driven and the near infrared light with the wavelength of, for example 850 nm is emitted from it. In this manner the light source control device 55 drives the light sources LD1 and LD4 sequentially with a time-sharing method.
Referring again to FIG. 5, the transmission light detection device 54 is equipped with a filter 57 which adjusts the quantity of near infrared light outputted to lenses 70 and 71 from the optical fiber 53; a photomultiplier tube 58 which converts the light from the filter 57 into pulse current and outputs it; an amplifier 59 which amplifies the pulse current from the photomultiplier tube 58; an amplitude discriminator 60 which eliminates the pulse current from the amplifier 59 whose amplitude is smaller than the prescribed threshold value; a multi-channel photon-counter 61 which detects photon frequency in every channel; a detection controller 67 which controls detection periods of the multi-channel photon-counter 61; and a temperature controller 68 which controls the temperature of a cooler 69 containing the photomultiplier tube 58.
To use the above-described examination apparatus, the illumination-side fixture and the detection-side fixture are firmly fitted to the prescribed positions of the patient's head 40 by using tape or the like. Once fitted, the light sources LD1 to LD4 are driven by the light source control device 55 as shown in FIGS. 3(a) to 3(d), respectively, so that the four near infrared light rays with different wavelengths are emitted from the light sources LD1 to LD4 sequentially with the time-sharing method and the light rays are introduced by the optical fibers 50-1 to 50-4 to the patient's head 40. As bones and soft tissues in the patient's head 40 are transparent to the near infrared light, the near infrared light is partially absorbed by hemoglobin in blood and cytochrome a, a.sub.3 in cells and outputted to the optical fiber 53. The optical fiber 53 introduces the light to the transmission light detection device 54. At the phase .phi.n1 no light source of LD1 to LD4 is driven, and therefore, the transmission light detection device 54 detects dark light.
The photomultiplier tube 58 in the transmission light detection device 54 is used with a photon-counting device that has high sensitivity and operates at high response speed. The output pulse current from the photomultiplier tube 58 is sent to the amplitude discriminator 60 through the amplifier 59. The amplitude discriminator 60 eliminates the noise component whose amplitude is smaller than the prescribed amplitude threshold and sends only the signal pulse to the multi-channel photon-counter 61. The multi-channel photon-counter 61 detects photons only in the periods T.sub.o. The periods T.sub.o are synchronized with the driving signals ACT1 to ACT4 for the respective light sources LD1 to LD4 as shown in FIGS. 3(a) to 3(d) by a control signal CTL as shown in FIG. 3(e). The control signal CTL is generated by the detection controller 67. The multi-channel photon-counter then counts detected photons of every light with each wavelength sent from the optical fiber 53. The transmission data of every near infrared light with each wavelength are obtained through the above-described procedure.
That is, as shown in FIGS. 3(a) to 3(e), at the phase .phi.n1 in the cycle CYn of light source control device 55 no light source of LD1 to LD4 is driven, therefore the dark light data d are counted by the transmission light detection device 54. At the phases .phi.n2 to .phi.n5 the light sources LD1 to LD4 are sequentially driven with the time-sharing method and the transmission light detection device 54 sequentially counts the transmission data t.lambda..sub.1, .lambda..sub.2, t.lambda..sub.3 and t.lambda..sub.4 of the respective near infrared light rays with different wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4.
The counting of the dark light data d and the transmission data t.sub..lambda.1, t.sub..lambda.2, t.sub..lambda.3 and t.sub..lambda.4 which is sequentially performed in the cycle CYn, is continued N times from CY1 to CYn. That is, one measuring period M.sub.k (k=1, 2, . . . ) includes N cycles. A concrete example is as follows; if one cycle is 200 .mu.sec and N is 10000, the measuring period M.sub.k becomes 2 sec. At the time of finishing of one measuring period M.sub.k, the counting result of the dark light data ##EQU1## and the counting results of the transmission data ##EQU2## are transferred to the computer system 56 and stored in the memory 63.
The processor 62 performs the subtraction of the dark light component by using the combination of the transmission data and the dark data (T.sub..lambda.1, T.sub..lambda.2, T.sub..lambda.3, T.sub..lambda.4, D)M.sub.k being stored in the memory 63 after one measuring period M.sub.k and the combination of those (T.sub..lambda.1, T.sub..lambda.2, T.sub..lambda.3, T.sub..lambda.4, D)M.sub.o at the start of measuring, and calculates the variation rates of the transmission light .DELTA.T.sub..lambda.1, .DELTA.T.sub..lambda.2, .DELTA.T.sub..lambda.3 and .DELTA.T.sub..lambda.4. That is, the variation rates of the transmission light .DELTA.T.sub..lambda.1, .DELTA.T.sub..lambda.2, .DELTA.T.sub..lambda.3 and .DELTA.T.sub..lambda.4 are calculated as: EQU .DELTA.T.sub..lambda.j =log [(T.sub..lambda.j -D).sub.Mk /(T.sub..lambda.j -D).sub.Mo ](j=1 to 4). (1)
The use of logarithm in the above calculation of .DELTA.T.sub..lambda.j is to express the variation as an optical density.
Using the above-calculated variation rates of the transmission light .DELTA.T.sub..lambda.1, .DELTA.T.sub..lambda.2, .DELTA.T.sub..lambda.3 and .DELTA.T.sub..lambda.4, density variations of oxygenated hemoglobin (HbO.sub.2), disoxygenated hemoglobin (Hb), oxidized cytochrome a, a.sub.3 (CyO.sub.2) and reduced cytochrome a, a.sub.3 which are expressed as .DELTA.X.sub.HbO.sub.2 .DELTA.X.sub.Hb, .DELTA.X.sub.CyO.sub.2 and .DELTA.X.sub.Cy, respectively, can be determined. That is, each of density variations of .DELTA.X.sub.HbO.sub.2 .DELTA.X.sub.Hb, .DELTA.X.sub.CyO.sub.2 and .DELTA.X.sub.Cy is calculated as: ##EQU3## where .alpha..sub.ij is an absorption coefficient of each component i (HbO.sub.2, Hb, CyO.sub.2, Cy) for each Wavelength .lambda..sub.j (.lambda..sub.1, .lambda..sub.2, .lambda..sub.3, .lambda..sub.4) and is predetermined from FIGS. 1(a) and 1(b), and l is the length of the patient's head 40 along the travelling direction of the near infrared light.
As the above-detected density variation components, .DELTA.X.sub.HbO.sub.2, .DELTA.X.sub.Hb, .DELTA.X.sub.CyO.sub.2 and .DELTA.X.sub.Cy, reflect the variation of oxygen quantity in the brain, the variation of oxygen quantity in the brain can be determined by outputting these detected results from the output device 64. The diagnosis is thus made based on these results.
The transmission quantity of the near infrared light greatly varies in the order of 10.sup.4 to 10.sup.5 with the size of the head 40, that is, the length l of the head 40 in the traveling direction of the near infrared light. Even if it is assumed that the size of the head 40 is kept constant being independent of the patient, as the output powers of the light sources (laser diodes) LD1 to LD4 vary in the order of 10.sup.1 to 10.sub.2 with their wavelengths of .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 (775 nm, 800 nm, 825 nm and 850 nm, respectively), the transmission quantities also vary sequentially in the order of 10.sup.1 to 10.sup.2.
On the other hand, it is desired that the light quantity which is made incident on the photomultiplier tube 58 in the transmission light detection device 54 during the measurement should be kept almost constant being independent of the length l of the head 40 and the variation of the output powers of the light sources LD1 to LD4 with the wavelengths, because the dynamic range of the photomultiplier tube 58 is approximately in the order of 10.sup.2.
Therefore, in the conventional examination apparatus 45, a variable light-attenuating ND (Neutral-Density) filter is employed as the filter 57, whose transmission factor can be manually adjusted. When the examination of one object person is started, the transmission factor of the filter 57 and each of the output powers of the light sources LD1 to LD4 are manually adjusted so that the incident light quantity on the photomultiplier tube 58, that is, the transmission light quantity becomes an optimum value.
As described above, the conventional examination apparatus has a problem that because of the manual adjustments of the filter 57 and the output powers of the light sources LD1 to LD4 it is difficult to adjust the transmission light quantity to the optimum value quickly and accurately. In addition, when the near infrared light transmitted from the head 40 is too intense the transmission quantity information incident on the photomultiplier tube 58 is cut off through attenuating the transmission quantity by the filter 57, which prevents the improvement of the measurement accuracy.
Another problem is that as the transmission factor of the filter 57 is kept constant after adjustment at the start of measurement, the variation of the transmission factor during the measurement caused by the position change of the filter 57 can not be restored. This also prevents the accurate measurement.